So your not going to take the advice to not mod. For a serious Autocross car off the shelf sport springs are WAY to soft you will want springs that are 200-300% stiffer that stock, but then you will need serious shocks to handle that high of spring rate. Plan on changing spring rates a few time before you get it right unless you can copy someone else that has already done it. Is your driving up to the point that you can tell if the cars wrong, or are you going to be chasing your setup when driving is the problem?
Drive the car for a year, maybe add some big sway bars and call it a day. I am a big fan of big sway bars.